As an inkjet recording liquid, a solution of a water-soluble dye such as an acid dye, a direct dyeor a basic dye in a glycol-containing solvent and water has been and is used in most cases (JP-A-53-61412, JP-A-54-89811 and JP-A-55-65269). As a water-soluble dye, generally, a dye having a high solubility in water is used for achieving the stability of a recording liquid. As a result, an inkjet print is generally poor in water resistance, and has a problem that a printed portion easily blots when water is spilled thereon.
For overcoming the above poor water resistance, attempts have been made to modify the structure of a dye or to prepare a highly basic recording liquid (JP-A-56-57862). Further, attempts have been also made to utilize a reaction between a recording sheet and a recording liquid for improving the water resistance (JP-A-50-49004, JP-A-57-36692, JP-A-59-20696 and JP-A-59-146889). These methods produce remarkable effects as far as specific recording sheets are concerned. However,these methods are not generally used since a limitation is imposed on the recording sheet. Further, when a recording sheet other than the specific ones is used, a recording liquid containing a water-soluble dye mostly often gives prints having no sufficient water resistance.
Further, as a recording liquid having good water resistance, recording liquids obtained by dispersing or dissolving oil-soluble dyes in solvents having high boiling points are available. However, these recording liquids have problems in the odors of solvents and the discharge of solvents and they are therefore environmentally undesirable. Further, another problem is that it is required to recover the solvents when recording is conducted in a large volume or in some place where a recording apparatus is located.
It is therefore being attempted to develop recording liquids which are dispersions of pigments in water-based solvent, for improving a printed product in water resistance.
However, as a printer using an inkjet recording liquid is desired to achieve a high resolution, the diameter of a nozzle decreases, and it is accordingly required to finely mill a colorant to decrease the particle diameter of the colorant.
Further, in forming an image on a transparent substrate for an overhead projector, it is demanded to achieve transparency equivalent to that of a dye, and finely milling is also required in view of the color development of a pigment.
Generally, as the dispersibility of a coating composition, an ink or the like is increased by decreasing the diameter of a pigment, the transparency improves. However, when a general dispersing machine such as a sand mill, a three-roll mill or a ball mill is used to carry out dispersing up to primary particles, the transparency no longer improves.
That is mainly because secondary particles are simply broken into primary particles in the dispersion step using the general. dispersing machine, and it is required to further finely mill the primary particles for further improving the transparency. A high-speed sand mill may sometimes further finely mill the primary particles of some pigments. However, it is practically very difficult to further finely mill the primary particles even if very large energy is applied.
As means of milling primary particles, there is known a method in which a solution of a pigment in a strong acid such as concentrated sulfuric acid or polyphosphoric acid is added to cold water to precipitate the pigment in the form of fine particles. In this method, however, the pigments that can be used are notably limited due to the solubility and stability of them in/to the strong acid. Further, since a pigment finely milled by the above method undergoes strong secondary aggregation when dried, it is very difficult to re-disperse the dried pigment up to the level of primary particles.
There is known another method in which a pigment and a solid resin are vigorously kneaded under heat with a two-roll mill or a Banbury mixer. Since, however, a pigment generally undergoes a crystal growth at a high temperature, the milling comes to an end when the milling force and the crystal growth are brought into an equilibrium state, and the milling by the above method has a limitation.
Further, an attempt has been also made to mechanically knead and mill a mixture of an organic pigment such as quinacridone or fused azo, a water-soluble inorganic salt and a water-soluble solvent at a high temperature of at least 80.degree. C. (JP-A-8-231780). In this method, particles having uniform forms can be obtained, while no intended finely milled pigment particles can be obtained due to similar crystal growth.
It is an object of the present invention to overcome the above problems of the conventional methods and to provide a water-dispersed inkjet recording liquid which is excellent in water resistance and transparency and has excellent stability in ejection from a nozzle, and a process for the production thereof.